Apparatus and circuits for preventing side tone



J. F. TOOMEY APPARATUS AND CIRCUITS FOR PREVENTING SIDE TONE April 221924.

Filed May 12 1919 EEE wg m wimwm cjalYw TINVEN ATTORNEY Patented Apr.22, 1924.

UNITED STATES PATENT OFFICE.

JOHN F. TOOMEY, NEW YORK, N. Y., ASSIGNOR TO AMERICAN TELEPHONE ANDTELEGRAPH COMPANY, A CORPORATION OF NEW YORK.

APPARATUS AND CIRCUITS FOR PREVENTING SIDE TONE.

To all whom it may concern:

Be it known that I. JOHN F. TOOMEY, residing at New York in the countyof New York and State of New York, have I invented certain Improvementsin Apparatus and Circuits for Preventing Side Tone, of which thefollowing is a specification.

Side tone is caused by the flow of current from the transmitter of atelephone sets.

Side tone is caused by the flow of circuits from the transmitter of atelephone set through the receiver thereof. It is objectionable becausethe user of a telephone 1| hears in the receiver not only the voice ofthe person with whom he is engaged in conversation. but also his ownvoice, as well as noises reaching his transmitter from adjacent sources.One well known manner in which side tone may be eliminated consists inbalancing the line by a device having like impedance and shunting thereceiver across the line so that, like the galvanometer of a balancedWheatstone bridge, it receives no current from the transmitter of thesame set. The balancing impedance is fixed in value and generallydesigned to balance the average subscribers line. Although thisarrangement is satisfactory for sets which are usually connected tolines of about equal length and impedance. it is ineffective to preventconsiderable side tone in sets which are frequently connected to linesof widely difiering impedance characteristics, and the side tone isparticularly objectionable when such sets are located in noisy places,such as central ofiices. The set of a toll tandem operator, for example,is particularly subject to noise for when this set is balanced for thelocal call circuits it is far out of balance for toll call circuits, andvice versa.

To reduce the side tone under such conditions, this invention providesapparatus and circuits whereby the balancing impedance is variedautomatically according to the impedance of the line with which the setis associated.

This invention may be readily understood by considering the followingdescri tion, with reference to the accompanying rawing which, indiagrammatic form. shows one arran ement of the apparatus and circuitsembodying this inventlon.

In this drawing reference character A designates a long distance callcircuit nor- Application filed May 12, 1919. Serial N01 296,475.

mally associated by means of a relay 3 with switchboard conductors 1 and2. In series with these conductors is connected the secondary winding ofa transformer 4 and an impedance device 5. A telephone transmitter 6 ofan operators set may be associated with the primary winding oftransformer 4 by means of the tip contacts of jacks 8. The current forthe transmitter is supplied by a battery 9 in series with a reactancecoil 10, and a condenser 11 is shunted across the battery and theretardation coil, as usual, to afford a path of low impedance to thevoice currents. The receiver 7 may be bridged across conductor 2 and themidpoint of secondary winding of transformer 4 by means of the sleevecontacts of the jacks 8. The impedance of the device 5 is approximatelye cal to that of the line A so that the drop of potential therein issubstantially equal to the drop in the line. The points at which thereceiver 7 is connected to the circuit are therefore of equal potentialrelative to current impressed on conductors 1 and 2 by transformer 4 sothat none of the output current of the transmitter enters the receiver.In other words, receiver 7 is conjugate to transmitter 6.

Reference characters B and C designate local call circuits differing inlength from each other and from circuit A, which call circuits may beassociated with conductors 1 and 2 by means of keys 15 and 16.respectively. These circuits are of much lower impedance than the tollline A and impedance 5 is too lar e for a proper balance. To lower theimpedance of the device 5 there are provided impedances 1'? and 18 whichmay be connected in parallel with impedanoe 5' by means of relays 19 and20 respectively. These relays are connected in series with relay 3, thecircuit of relay 19, for instance, extending from battery 22, throughthe winding of relay 3, conductor 23, relay 19, conductor 24, contact 25of key 16 to ground. When this key is operated battery 22 energizesrelays 3 and 19 and causes the former of these to disconnect line A fromconductors 1 and 2 and apply a source of tone current to the line toindicate that the operator is connected to another call circuit. Therelay 19 also closes its contact and thus connects impedance 17 inparallel with the impedance 5. The i1npedance 17 of such value that thecombined impedanceof devices 17 and 5 in parallel is approximately equalto that of line C. Conjugacy between receiver 7 and transmitter 6 isthus maintained. The circuit of relay 20 is the same as that of relay 19except that it extends through the wire 27 to ground at contact 28 onkey 15, so that this relay is operated in conjunction with relay 3 whenthe operator manipulates key 15. Relay 20 connects impedance 18 inparallel to impedance 5, the impedance of device 18 being of such valuethat the parallel impedance of devices 5 and 18 is substantially equalto that of line B. Conjugate relationship between receiver 7 andtransmitter 6 is thus maintained when the telephone set is associatedwith circuit B.

It is readily understood that, although only three lines are shownherein, any number of other lines of different impedances may beassociated with the set by additional keys and connections, and thatproper balance for each of these may be maintained by other impedancescontrolled by other relays. It should further be noted that all thelines of approximately like impedance may have the same balancingimpedance device associated therewith. Thus it is rendered possible bythis invention to obtain for each line sufficiently close balance torender the side tone practically ineffective to interfere with theconversation.

Although only one form of embodiment of the invention is shown anddescribed herein it is readily understood that various changes andmodifications may be made therein Within the scope of the followingclaims without departing from the spirit and scope of the invention. 1

What is claimed is:

1. In combination, a plurality of telephone lines. a telephone set, animpedance device associated with said set and adapted to be connected inseries in one of said lines. and means for associating said set with anyone of said lines and adjusting the impedance of said impedance deviceaccording to the impedance of the line with which said set isassociated.

2. In combination, a plurality of telephone lines, a telephone setnormally connected to one of said lines, an impedance device normallyassociated with said set for halacing currents from the transmitter ofsaid set to the said lineand associated impedance, in respect to theefi'ect of said currents on the receiver. and means for selectivelyassociating the set with another of said lines instead of with said oneline, and for changing the impedance of said device to accord therewith.

3, In combination, a telephone set. a plurality of telephone lines oneof which is normally connected with said set. switching devices forselectively associating any other said line with said set, an impedancedevice corresponding to said normally connected line, said impedancedevice being normally connected to said telephone set, supplementalimpedance devices adapted to be connected to modify the impedanceafforded by the first mentioned impedance device, relays for saidsupplemental impedance devices for associating them with the lines tobalance the currents from the transmitter of said telephone set to theline and associated impedance, with respect to the effect of saidcurrents on the receiver of said set, and circuits for said relaysgoverned by the said switching devices.

4.111 combination, a plurality of telephone lines, a telephone set. afirst relay normally associating one of said lines with said set, animpedance device for balancing currents from the transmitter of saidtelephone set to the line and associated impedance, in respect to theeffect of said currents on the receiver of said set, a switch ing devicefor associating another of said lines with said set, a second impedancedevice, a second relay for associating said second device with said setin balancing relationship and a circuit for governing said first andsecond relays by said switching device.

5. In combination, a telephone line, an impedance device associatedtherewith for balancing the impedance of the line, a tele phone sethaving its recevier and transmitter associated with said lineapproximately in conjugacy, a second telephone line, means fordissociating said first line from said set and associating said secondline therewith, and means for changing the impedance of said device intosubstantial balance with said second line.

6. In combination, a telephone line, an impedance device associatedtherewith for balancing the impedance of the line. a telephone sethaving its receiver and transmitter associated with the lineapproximately in conjugacy, a second telephone line. a switching devicefor associating said second line with said set, a relay for dissociatingsaid first line from said set. a correcting impedance device, a relayfor associating said correcting impedanc device with said firstimpedance device and a circuit for controlling said relays by saidswitching demac.

7. In combination. a telephone set comprising a transmitter and areceiver associated in conjugate relationship, a plurality of telephonecircuits differing in impedance. means for selectively associating saidset with said circuits, and means for retaining said transmitter andreceiver in substantial conjugacy irrespective of the telephone circuitwith which the set is associated.

8. In combination, a transmitter, 11. re-

ceiver, a circuit comprising a line and a local impedance, in whichcircuit said transmitter and receiver may be connected in con jugaterelationship, and means to change their connection to a line ofdiflFerent electrical characteristics and at the same time change thelocal impedance so as to preserve the conjugate relationship of saidtransmitter and receiver.

9. In combination, a transmitter, a receiver, a plurality of lines, anadjustable impedance, and means to connect said transmitter and receiverwith any one of said lines and at the same time to adjust the saidimpedance. said connection and adjustment being such that the saidtransmitter and receiver are in all cases in conjugate relationship.

10. In combination, a plurality of telephone lines, a telephone set, animpedance device associated with said set and adapted to be balancedagainst the line for currents from the transmitter of said set, andmeans for associating said set with any one of said lines andsimultaneously adjusting the impedance of said impedance deviceaccording to the impedance of the line with which said set isassociated.

11. In combination, a plurality of telephone lines, a telephone sethaving its transmitter and receiver associated with one of said lines inconjugate relationship, an impedance device in the line, and means forassociating said set with any other line and adjusting the impedance ofsaid impedance device according to the impedance of the line with whichsaid set is associated.

In testimony whereof I have signed my name to this specification this8th day of May, 1919.

JOHN F. TOOMEY.

